Thanks to the state of California, smartphone apps will soon have to tell you in advance what they plan to do with your personal data.

California Attorney General Kamala D. Harris reached an agreement with Apple, Google, Microsoft, Research in Motion and HP–that’s every company that has a smartphone or tablet platform in the United States–to require privacy policies for mobile apps. That means third-party apps, such as Path and Instagram, will have to warn users ahead of time if the apps will store address books on their servers or share personal data with marketers, among other things.

Harris told USA Today that she’ll review compliance in six months. “We are assuming everyone is going to cooperate in good faith and not get cute,” she said.

Still unclear, though, is how companies like Apple and Google will enforce the rules. When contacted by the Wall Street Journal, none of the companies involved would elaborate on their plans.

But my guess is that app makers will have to put some kind of privacy policy on their app store landing pages in the same space as screenshots, written descriptions and user reviews. Another possibility is some kind of pop-up privacy warning before the download commences, but that seems less likely because it would risk scaring off customers.

Of course, users may ignore these privacy policies anyway, just as they tend to skip over verbose end user license agreements. And posting a privacy policy won’t directly prevent app makers from running amok with personal data. But a little transparency is better than nothing.

Although the rules only apply in California, they’ll affect all smartphone and tablet users in the United States.